Millimeter-wave frequency bands hold valuable spectrum for what lies ahead: fifth-generation (5G) wireless communications and automotive collision-avoidance radar systems. Signals at 60 GHz and higher might have once been thought too high to transmit and receive with affordable circuits. But semiconductor devices and circuit technologies have improved in recent years and millimeter-wave circuits are becoming standard electronic equipment in many car models. Millimeter-wave signals are also expected to play major roles in 5G networks in transferring high-speed data over short distances. For that to happen, low-loss laminates must be available for circuits operating from 60 through 77 GHz, without performance limitations placed by the glass weave effect at those high frequencies. Just what is the “glass weave” effect and what does it have to do with millimeter-wave circuits? It’s all about the wavelengths.

Glass and fiberglass fabrics are commonly used to fortify resin-based circuit laminates. Many PCB materials for higher-frequency use are formed from different woven glass fabrics bound together with epoxy resins. The glass fabrics actually follow precise patterns through the PCB material, with a warp yarn running the length of the material and a fill yarn running the width of the material. The relative permittivity (Dk) values of these different material components are different, so the combination of glass fabrics and epoxy resins form a non-homogeneous medium for signals propagating through transmission lines formed on that medium.

Although such non-homogeneity is less of a concern at lower, RF signals, for millimeter-wave signals with extremely small wavelengths, differences in Dk throughout a propagation medium can result in differences in the characteristic impedance of transmission lines fabricated on that medium. The epoxy resin typically has a lower Dk value than the glass fabric, and the density of the glass fabric will change throughout a PCB as a function of the glass weave pattern. Quite simply, where there is more glass, there is a higher Dk value. Depending upon a particular glass weave, glass bundles can form, resulting in a rise in the Dk value at that location of the PCB material.

In terms of example values, the Dk of a typical resin system may range from 2.0 to 3.0 while the Dk of the glass bundles formed by the glass weave running through the material can be equal to 6.0 or higher. In the open areas of the PCB between glass bundles, the Dk of the laminate will be much lower in value than in those areas around the glass bundles. For lower-frequency signals with relatively large wavelengths, a certain amount of averaging of the effective Dk values of these different sites will take place, resulting in fairly predictable signal propagation behavior that can be accurately analyzed with a computer-aided-engineering (CAE) software simulation program. But at higher, millimeter-wave frequencies, where the signal wavelengths are smaller, the differences in Dk across the PCB due to the glass weave effect can result in transmission-line impedance differences that cause phase shifts at millimeter-wave frequencies.

The types of transmission line used in a high-frequency circuit can also play a part in how significant the role of the glass weave effect plays on the performance of a millimeter-wave circuit. In a multilayer microstrip circuit, for example, due to the randomness of the glass fabric patterns from layer to layer, it is likely that a certain amount of averaging in the Dk will occur across the circuit board and more consistent performance will be achieved in a multilayer circuit construction. Any type of circuit construction in which two or more layers with glass weave are used will benefit from the averaging effects of the multiple layers.

High-speed digital signals such as differential lines operating at data rates beyond 10 Gb/s can be affected by the increased concentrations of glass bundles within PCB material, since the differential lines depend upon tightly maintained phase relationships for their signal information. As with millimeter-wave signals, high-speed differential lines rely upon circuit materials with low conductor and dielectric losses; minimizing signal phase variations as a result of the glass weave effect is a positive circuit material trait for both millimeter-wave and high-speed-digital signal propagation.

Admittedly, the glass and fiberglass fabrics that are combined with the resin systems to form high-performance circuit materials provide a great deal of mechanical strength to the circuit material, although the non-homogeneity that they can introduce to the material at higher frequencies can be an unwanted side-effect at millimeter-wave and high-speed-digital signals. Automotive radar systems, for example, rely upon the reception of reflected pulses at 77 GHz to determine the position of other vehicles in traffic as well as pedestrians. Phase variations resulting from transmission-line skew in a PCB can effectively shift the position of vehicles being detected in traffic.

Fortunately, the benefits of glass material reinforcement can be added to high-frequency circuit laminates without suffering the negative impact of the glass weave effect. Newer circuit materials such as RO4830™ circuit laminates from Rogers Corp. combine glass and resin materials with a type of glass known as “spread glass.” Rather than using a bundled configuration with a tendency to produce uneven distribution of the glass content throughout the laminate, the glass material is spread evenly throughout the epoxy resin, with no openings between the glass bundles. In this way, the layer of glass fabric in the laminate appears very much like a plane of glass, minimizing or eliminating any variations in Dk throughout the laminate.

RO3003™ circuit laminates from Rogers Corp. are low-loss, ceramic-filled, PTFE-based laminates engineered for circuits to 77 GHz and beyond. This laminate does not have woven-glass fabric and therefore has no concern with the glass-weave effect. The laminate features a Dk of 3.00 ± 0.04 across the board for extremely consistent and predictable performance even at millimeter-wave frequencies. These materials have additional characteristics that make them a good fit for millimeter-wave circuits, including very low moisture absorption, nearly ideal thermal coefficient of Dk (TCDk) at 3 ppm/ºC, and a coefficient of thermal expansion (CTE) of 17 ppm/ºC that is closely matched to copper in the x and y axes and equal to 24 ppm/ºC in the z-axis for highly reliable plated through-holes.

For any concerns related to the glass weave effect, RO4830 materials are produced by means of the spread glass approach, thus avoiding the potential for glass bundles from the glass weave effect. RO4830 and RO3003 materials provide the mechanical stability with temperature to maintain consistent low-loss performance even in rigorous automotive operating environments and, as expected, for an emerging number of 5G millimeter-wave data link applications.

This ROG Blog series on printed-circuit-board (PCB) materials has reached the half-century mark, already covering a wide range of topics on circuit materials with this, the 50th ROG Blog. For example, this series has recommended materials for amplifiers, for antennas, for filters, and for different types of transmission lines. It has even detailed the effects of different PCB material thicknesses on circuit performance, and described the influence of conductor roughness on circuit performance.

While it would be difficult to pick out the top 10 Blogs from the first 49 Blogs appearing since August 2010, at least 10 of these ROG Blogs deserve mention for how they have attempted to help readers with their different uses of PCB materials.

While it would be difficult to name the “Top Ten” ROG Blogs from the series so far (see the list below), it is not surprising to find that one of the most popular (in terms of viewers/readers) would be one that also refers to something for free: the January 2011 ROG Blog on Rogers’ free transmission-line modeling tool, the MWI-2010 Microwave Impedance Calculator. This easy-to-use modeling tool, which has also been reviewed in many of the leading RF/microwave trade publications, calculates key parameters for most common microwave transmission lines, including microstrip, stripline, and coplanar-waveguide transmission lines. The executable (.exe) file is available for free download from the Rogers’ website and runs on Windows-based personal computers (PCs), including those with Windows XP, Windows Vista, and Windows 7 operating systems. The free software is even backed by a 22-page operator’s manual in PDF file format, also available for free from the Rogers website.

In many ways, the ROG Blog series is like a book on circuit materials, unfolding online before its readers, with each Blog adding a new chapter to the book. Each chapter shares what Rogers’ engineers have learned over the years about making and using circuit materials, and this first set of 50 Blogs has covered some areas of interest to a large number of readers. In line with the ROG Blog on free software, the ROG Blog “Comparing RF Circuit Material Processing Costs & Performance” also offers advice meant to help readers save money without sacrificing their performance goals. Although first appearing on “April Fool’s Day” (April 1) in 2011, this ROG Blog takes a serious look at the total costs of circuit materials, and how some circuit materials may have lower material costs than other materials, but pay for it later with higher processing costs and lower yields. It also explains how some performance parameters, such as passive intermodulation (PIM) in wireless circuits and signal integrity in digital circuits, require a careful consideration of tradeoffs in material and processing costs when choosing a circuit material.

These first 50 ROG Blogs have drawn readers for familiar themes as well as for some not-so-familiar topics. For example, the ROG Blog appearing on November 19, 2010, “What Is Outgassing and When Does It Matter,” addresses a subject that may be unknown to some readers but quite significant to others. Outgassing, which refers to the release of gas inside a solid such as a circuit material, especially when it is placed in a vacuum, can greatly impact the performance of circuits used in satellite-communications systems in space, or in medical electronics systems. This ROG Blog introduced many readers to a material term known as total mass loss (TML), and how the parameter could be used to help guide the selection of a circuit material for space-based or other applications where outgassing was a critical concern.

On the other hand, some of the more popular ROG Blogs covered the roles that circuit materials play in the design of some basic RF/microwave components, such as amplifiers, couplers, and filters, and how the choice of a circuit material can affect transmission-line losses in high-frequency circuits. One of the more popular ROG Blogs, “When Digital Signals Reach Microwave Frequencies,” covered an area of interest to many microwave circuit designers, how to deal with digital circuits operating at microwave frequencies. This ROG Blog, appearing on February 23, 2011, reviews some of the important concerns for selecting a circuit material when circuits cross over from the digital area into the microwave realm. These high-speed digital signals will behave much like analog microwave signals, affected by PCB loss and even conductor surface roughness. To guide those in need of circuit materials for high-speed digital designs or even multilayer circuits that may combine fast digital and microwave circuits, this ROG Blog points out how different circuit material characteristics, such as dielectric constant and even coefficient of thermal expansion (CTE), can impact high-speed digital circuit performance.

At times, readers of the ROG Blog series shared their areas of interest and applications for circuit materials, and these applications are many and diverse, from lower-frequency analog and power circuits to high-speed digital and even microwave/millimeter-wave circuits. The ROG Blog series is written to serve its readers with new information on circuit materials as that information is needed, much like new chapters to an on-going, online book about circuit materials. Do you have a suggestion for future ROG blogs? We’d love to get your input. Let us know what you are interested in reading about.

Millimeter-wave frequencies (about 30 to 300 GHz) were once associated with at least two things: circuits for these frequencies are extremely difficult to fabricate, and they will probably be used for some military-electronics application. However, the United States’ Federal Communications Commission (FCC), among other organizations around the world, is doing its part to free wide portions of bandwidth for unlicensed radio use at millimeter-wave frequencies.

The FCC is treating wide millimeter-wave bandwidths, such as the 7 GHz span centered at 60 GHz (57 to 64 GHz) as Industrial-Scientific-Medical (ISM) band frequencies so that they can be used for commercial and other unlicensed applications by the general public. Because these frequencies are available for use without licenses, a growing number of circuit designers are considering different applications at these higher frequencies and, of course, choosing the right printed-circuit-board (PCB) material is an important part of any practical efforts to realize millimeter-wave circuits.

Millimeter-Wave ISM Bands

Organizations such as the FCC have set aside a number of different millimeter-wave bands for unlicensed use in addition to 60 GHz, such as 94, 140, and 220 GHz. Receivers and transmitters at these frequencies are currently being produced in the form of integrated circuits (ICs) based on gallium arsenide (GaAs) and even silicon semiconductor processes, such as silicon CMOS and silicon germanium (SiGe) technologies. As the speeds of computers increases, and the demand for faster Internet access grows, the fast (better than 1-Gb/s) data rates available in these unlicensed millimeter-wave ISM bands makes the use of millimeter-wave links attractive for a variety of short-range communications links.

For example, fixed-frequency millimeter-wave wireless links offer the bandwidth possible with fiber-optic links, but with a fraction of the time and cost required to install a high-speed fiber-optic communications link. For this reason, millimeter-wave links are popular solutions for providing radio backhaul for cellular-communications base stations.

Of course, while government agencies around the world may be freeing millimeter-wave frequency bands for unlicensed use, the task of designing and fabricating circuitry at these elevated frequencies has not become any easier. This blog series has already taken a look at some of the circuit-material characteristics that can impact the performance of millimeter-wave circuits. For years, military-electronic systems have employed phased-array radar systems at millimeter-wave frequencies. And millimeter-wave frequencies have been used extensively in high-end automotive electronic systems, including for long-range adaptive cruise control at 77 GHz and anti-collision systems at 79 GHz. At higher frequencies, millimeter-wave circuits have been part of airport security and imaging systems at 94 GHz. But with increasing opportunities for millimeter-wave circuit applications, especially for communications at ISM bands, it may help to review some of the key circuit material considerations when working at millimeter-wave frequencies.

Circuit Material Considerations at Millimeter-Wave Frequencies

Wavelengths for signals from 30 to 300 GHz are extremely small, from about 1 cm to 1 mm. Although this translates into reduced circuit dimensions, it also makes possible the use of modest-sized antennas with focused beamwidths. As an example of the reduction in size that is possible at these higher frequencies, an antenna with 1-deg. beamwidth for a line-of-sight communications link at 3.5 GHz has a nominal diameter of 12 ft. But for a line-of-sight link at 60 GHz, an antenna with a 1-deg. beamwidth is a mere 8 in. in diameter.

In terms of millimeter-wave circuitry, it is important to remember the impact of various circuit parameters on performance at millimeter-wave frequencies. Circuit designers typically work with a material that is familiar to them based on such characteristics as dielectric constant and dissipation factor, using those parameters where possible in a computer simulation program to project the performance of a particular circuit configuration. Because the physical size of a high-frequency circuit transmission line is dependent on the dielectric constant of the PCB material, the value of the circuit material’s dielectric constant is particularly critical at millimeter-wave frequencies, where circuit dimensions can be so small.

For this reason, PCB materials with the lowest possible dielectric-constant values are to be preferred for millimeter-wave circuit applications. Circuit dimensions shrink with higher values of dielectric constant, but reducing the size of necessarily small circuit dimensions can make those circuits difficult to fabricate with consistency.

The consistency of the dielectric constant across a circuit board can also be an important concern at millimeter-wave frequencies since, at those frequencies, variations in the dielectric constant can introduce variations in the signal phase. For a given consistency of dielectric constant, the phase variations will increase with increasing frequency, and will hinder the performance of circuits that depend on reliable phase behavior, such as in phase-modulated communications systems and phased-array radar systems.

Millimeter-wave phase performance can also be affected by the composition of the PCB material. For example, circuit materials that are reinforced using a glass weave can exhibit phase-based problems when the glass weave is not consistent throughout the material. In a manner somewhat akin to an inconsistent dielectric constant, this can lead to perturbations in a circuit’s signal propagation velocity, which cause signal integrity issues, including uneven phase performance. The unwanted results can be distortions in phase modulation and errors between phase-matched channels in radar systems.

Minimizing Atmospheric Losses

Millimeter-wave communications links typically support high data rates at line-of-sight distances to about 1 km, but they are subject to atmospheric losses even for such short links. To minimize losses at those higher frequencies, PCB materials with the lowest possible dissipation factors should be used for millimeter-wave circuits. The quality of a PCB’s conductor surface can also play a role in loss performance at millimeter-wave frequencies. A rough copper surface will yield higher conductor losses at higher frequencies, so that selecting a PCB material with smooth copper conductor surface can help minimize loss at millimeter-wave frequencies.

The thickness of a PCB’s conductor layer can also be a concern at millimeter-wave frequencies because of a parameter known as skin depth. Skin depth refers to the thickness into the conductor material at which a propagating electric field has decreased by about 37%. Skin depth decreases rapidly with increasing frequency, at about 6.6 μm at 100 MHz, about 0.66 μm at 10 GHz, and about 0.2 μm at 100 GHz. With the small skin depth at millimeter-wave frequencies, it is easy to see the impact that copper conductor roughness can have on loss performance.

The thickness of the PCB material is also a consideration at millimeter-wave frequencies, since moding effects and unwanted resonances can result from the use of a thick circuit material with such small wavelengths. Circuit materials used for millimeter-wave circuits are typically in the thickness range of 2 to 10 mils. As they do with increasing dielectric constants, circuit line widths also shrink with thinner PCB materials. Since circuit loss decreases with the increasing thickness of circuit dielectric materials, selecting a PCB material for millimeter-wave applications is something of a tradeoff between creating stable, practical, and producible circuits and achieving low loss for those circuits.

Given this challenging set of requirements, what types of real-world materials are suitable for millimeter-wave circuits? Two examples are RT/duroid® 5880 and RT/duroid 5870 laminates from Rogers Corp. Both are PTFE-based composite materials with low dielectric constants, good consistency of dielectric constant, and low loss. RT/duroid 5880 laminate has a dielectric constant of 2.20 in the z direction at 10 GHz with dissipation factor of a low 0.0009 at 10 GHz. RT/duroid 5870 laminate has a dielectric constant of 2.33 in the z direction at 10 GHz with dissipation factor of 0.0012 at 10 GHz. These materials can be supplied in sheets as thin as 3.5 mils for excellent performance at millimeter-wave frequencies.

In summary, when working at millimeter-wave frequencies, circuit materials should ideally be electrically homogeneous, as thin as possible, with low dielectric constant, low dissipation factor, and with a smooth conductor surface.

Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.